Coking and carburization are problems in a variety of high temperature hydrocarbon conversion processes. Recently, new coatings have been disclosed that prevent carburization in some of these processes. For example, U.S. Pat. No. 5,405,525 to Heyse et al. discloses a method for reforming hydrocarbons comprising coating portions of a reactor system with a material more resistant to carburization, reacting the material with metal sulfides existing in the portions of the reactor system prior to coating, fixating and removing at least a portion of the sulfur in the metal sulfides, and reforming hydrocarbons in the reactor system under conditions of low sulfur.
Similarly, U.S. Pat. No. 5,413,700 to Heyse et al. discloses a method for reforming hydrocarbons comprising coating portions of a reactor system with a material more resistant to carburization, reacting the material with metal oxides existing in the portions of the reactor system prior to coating, fixating or removing at least a portion of the oxide in the metal oxides, and reforming hydrocarbons in the reactor system under conditions of low sulfur.
Also, U.S. Pat. No. 5,406,014 to Heyse et al. discloses methods for dehydrogenation of alkanes into alkenes in reactor systems of improved resistance to carburization under dehydrogenation conditions. The reactor walls are provided with a carburization and abrasion resistant protective layer by applying a metal plating, cladding or other coatings, such as painting, of a metal, such as Sb, As, Bi, Cu, Cr, Ga, Ge, In, Pb, Se, Te, Sn, particularly tin as a stannide layer, for forming a carburization resistant protective layer to a thickness of about 0.5 to 15 mils, effective to isolate the steel portion from hydrocarbons during the dehydrogenation process while avoiding any substantial liquid metal embrittlement. The protective layer is formed anchored to the steel portion through an intermediate carbide-rich bonding layer. The invention can be applied to conversion of ethylbenzene to styrene. This patent does not discuss changing the steam to hydrocarbon ratio.
Problems associated with carburization include coking, carburization of system metallurgy, and metal dusting. The embrittlement of the steel walls by carburization leads to "metal-dusting", i.e., a release of catalytically active particles and liquid droplets of metal due to an erosion of the metal. The excessive "metal-dusting" adds active metal particulates to the system, which particulates provide additional sites for coke formation.
One solution to the problems associated with carburization, embrittlement, and metal-dusting is to add steam and often sulfur as well to the feed to thereby effectively inhibit carburization. However, the addition of either steam or sulfur increases production cost and process complexity.
Nonetheless, some high temperature hydrocarbon conversion processes utilize steam, often in large amounts. These processes include thermal cracking of light and heavy hydrocarbons to ethylene and propylene; the conversion of ethylbenzene to styrene; and the steam reforming of hydrocarbons such as natural gas to hydrogen and other products, and the dehydrogenation of butene to produce butadiene.
In the production of ethylene by thermal cracking, a diluent fluid such as steam is usually combined with a hydrocarbon feed such as ethane and/or propane and/or naphtha, and introduced into a cracking furnace. Within the furnace, the feed stream which has been combined with the diluent fluid is converted to a gaseous mixture which primarily contains hydrogen, methane, ethylene, propylene, butadiene, and small amounts of heavier gases. At the furnace exit this mixture is cooled to remove most of the heavier gases, and then compressed. The compressed mixture is routed through various distillation columns where the individual components such as ethylene are separated and purified.
Steam serves a variety of purposes. It is a diluent used in order to improve yields. It drives the hydrocarbon through the system. In the case of ethylene plants, it oxidizes the steel somewhat, and inhibits metal-catalyzed coking on the furnace tube walls.
One recognized problem in the production of ethylene by thermal cracking is coke formation. Because coke is a poor thermal conductor, as coke is deposited, higher furnace temperatures are required to maintain the gas temperature in the cracking zone at necessary levels. Higher temperatures increase feed consumption and shorten tube life. Also, cracking operations are typically shut down periodically to burn off deposits of coke. This downtime adversely affects production.
Another problem in thermal cracking is the embrittlement of the steel walls in the reaction system. Such embrittlement is due to carburization of the system metallurgy, and ultimately leads to metallurgical failure. In King et al, "The Production of Ethylene by the Decomposition of n-Butane; the Prevention of Carbon Formation by the Use of Chromium Plating", Trans. of the E.I.C., 3, #1, 1 (1959), there is described an application of a 3/1000 inch thick (3 mil) chromium plate to a stainless steel reactor. This chromium plate is described as peeling-off the surfaces of the steel after a period of several months of operation, which was attributed to the high temperatures required for the reaction, and periodic heating and cooling. This peeling occurred in the absence of steam, which we have found can itself induce peeling under certain process conditions.
Although coking is a problem which must be addressed in process plants such as ethylene crackers and styrene plants, this problem is significantly aggravated in the absence of steam in reactor feedstreams. In fact, it is believed that in the absence of steam, active metal particulates in coke particles will metastasize coke generally throughout the system. That is, the active metal particulates actually induce coke formation on themselves and anywhere that the particles accumulate in the system resulting in coke plugs and hot regions of exothermic reactions. As a result, a premature coke-plugging of the reactor system occurs which can lead to a premature shut-down of the system.
Still another problem in processes that utilize steam, such as thermal cracking, is reduced hydrocarbon throughput. Although steam provides some of the thermal energy required for the cracking reaction, its presence in the cracking tubes necessarily displaces hydrocarbons. Often the amount of steam used is more than necessary to provide the needed thermal energy. If hydrocarbons could replace part or all of the reactor volume that is filled with process steam, hydrocarbon throughput could be increased. Thus, if a unit is not operating at or near the effective steam burner capacity, then less steam at a higher temperature could be used and product production could be increased.
Operating at lower steam levels would have other advantages. For example in ethylene plants, the production of light gases, such as CO and CO.sub.2 that have to be scrubbed from the product, would be reduced.
Another process that uses steam is the catalytic dehydrogenation of styrene from ethylbenzene. This process is generally catalyzed with an Fe.sub.2 O.sub.3 catalyst containing stabilizers and coke retardants. There are two major types of ethylbenzene to styrene processes: isothermal (older technology) and adiabatic. The isothermal process uses a fixed bed, shell and tube heat exchanger. The process runs at about 600.degree. C. outlet temperatures. The steam/hydrocarbon mole ratio is about 8:1 for newer plants and about 10:1 for old plants.
Steam is added to the ethylbenzene/styrene process for at least the following reasons:
to prevent coking and carburization of the metal surfaces PA1 to provide heat to the endothermic dehydrogenation reaction PA1 to inhibit catalyst coking and lengthen catalyst life PA1 to reduce hydrocarbon partial pressure and shift equilibrium towards styrene PA1 (i) providing a carburization, abrasion and peeling resistant and coking resistant Group VIB metal protective layer to a steel portion of a cracking reactor system by (a) applying to the steel portion a Group VIB metal plating, cladding or other coating of Group VIB metal effective for forming a carburization resistant protective layer, to a thickness effective to isolate the steel portion from hydrocarbons during operation, and (b) forming the protective layer, anchored to the steel portion through an intermediate carbide-rich bonding layer; and PA1 (ii) thermally cracking a hydrocarbon feed of ethane, propane and/or naphtha feed to produce ethylene, said process operated at low steam levels to increase hydrocarbon throughput. PA1 higher production rates, increased hydrocarbon throughput PA1 lower energy costs PA1 lower pressure drop across the catalyst bed PA1 lower capital costs
However, there are problems with current styrene operations. These include high energy costs associated with heating and cooling steam which result in a high cost of styrene production. A larger plant (per pound of styrene produced) is required to process steam; larger reactors, transfer piping, heat exchangers, etc. are required in the reactor section to handle the large volume of steam. In addition, water separation facilities are required to handle condensed steam downstream of the reactor section.
Another problem is coking in the feed/effluent heat exchanger in styrene plants. Ethylbenzene and steam are heated on the shell side of this exchanger. Coke builds up during normal operation and will eventually damage the exchanger. Premature steam loss (e.g., during a shutdown) initiates massive coke formation that can rapidly destroy the exchanger.
Still another problem is that ethylbenzene and styrene crack in the hot section of the plant to form undesirable by-products, which result in lower selectivity. These cracking reactions are believed to be both thermal and catalytic. Metal-catalyzed reactions on the metal surfaces of the reactor cause at least some of this cracking.
Consequently, there remains a need in the art for improved processes which utilize steam at elevated temperatures and where cracking and coking are reduced, especially at low steam levels. Such a method would include means for inhibiting the undesirable catalytic activity which causes catalytic cracking and coking, as well as means for inhibiting carburization of system metallurgy.
Thus, although there are problems and disadvantage associated with the use of steam, it is currently not possible to remove or reduce the amount of steam from the processes described above. That is, it was not possible to remove or reduce the amount of steam until the discovery of the present invention.
Accordingly, one object of the present invention is to reduce the amount of steam used in hydrocarbon conversion process which currently utilize steam. Another object is to increase the hydrocarbon throughput in these processes. These and other objects will be evident from the description of the invention which follows.